It is generally assumed that widespread "depression" of the genome occurs in the cells of the regenerating liver in preparation for replication. However, no detailed studies of genome expression during liver regeneration are available at this time. We have used molecular hybridization techniques to examine the pattern of transcription and the intracellular distribution of unique and repetitive sequence transcripts in regenerating rat liver. The experiments include a) RNA-DNA hybridization in RNA excess, using H3-RNA labeled with H3-dimethylsulphate in vitro; b) hybridization of poly A ion polysomal RNA with highly labeled unique sequence DNA; c) synthesis of complementary DNA using polysomal poly A ion RNA from normal and regenerating livers as template; d) hybridization of cDNA with whole or unique sequence DNA. Our results to date show that at least for repetitive sequence transcripts, massive "derepression" of the genome does not occur after partial hepatectomy. BIBLIOGRAHIC REFERENCES: Fausto, N. and Butcher, F.R. - Cyclic Nucleotide Levels in Regenerating Liver. Biochem. Biophys. Acta, in press. Greene, R.F. and Fausto, N. - Transcriptional patterns during liver regeneration. J. Cell. Biol. 67, 145a, 1975.